How can I build a foundation of faith so that I may remain steadfast in religion after repentance?

Among the things that help you repent and build a foundation of creed are: constantly contemplating the names and attributes of the Lord, being mindful of His encompassing knowledge of all things, the severity of His punishment for transgressors, and the vastness of His mercy for those who repent. Also, being mindful that resurrection is a reality, that the Hour is coming, undoubtedly, that Allah will raise those in the graves, and that everyone will stand before Him and be held accountable for all their deeds. We advise you to read "Majmoo'at al-'Aqeedah" (The Creed Collection) by Sheikh Dr. Omar Al-Ashqar.